The present invention relates to an ion current sensing device for an internal combustion engine which can accurately sense an ion current generated between electrodes of a spark plug during combustion of a mixture in an engine cylinder for an extended period of engine operation.
FIG. 2 is a schematic diagram showing the general arrangement of an ignition device in an internal combustion engine equipped with a known ion current detecting device. The ignition device illustrated includes an ignition coil 1 having a primary winding and a secondary winding which is connected to a computerized engine control unit (ECU) 3 through a switch element 2 in the form of a power transistor. The power transistor 2 has a collector coupled to the primary winding of the ignition coil 1, a base coupled to the ECU 3 which controls the ignition timing of an engine cylinder 7, and an emitter grounded. The secondary winding of the ignition coil 1 is connected to a cathode of a reverse-current checking diode 4 whose anode is connected to a spark plug 5 which is mounted on the head of the engine cylinder 7 in which a piston 7a is received for reciprocating motion. The spark plug 5 has electrodes 5a thereof present in a combustion chamber 6 defined in the engine cylinder 7 for igniting an air/fuel mixture therein. A signal generator 8 senses the crank angle or rotational position of the piston 7a or a crankshaft 18 connected therewith and generates a corresponding crank angle signal in synchronism with the rotation of the crankshaft 18. An ion current sensing device, which is generally designated by reference numeral 9, is connected at an input terminal thereof through a diode 16 to a junction between the reverse-current checking diode 4 and the spark plug 5 and at an output terminal thereof to the control unit 3. The ion current sensing device 9 includes a signal processor 10 which is connected at its output to the output terminal of the device 9 which is in turn connected to the ECU 3, and a comparator 11 which has a first input terminal connected to a junction A between the cathode of the diode 16 and one end of an ion current sensing resistor 15 whose other end is connected to a negative terminal of a DC power source 14, and a second input terminal on which a reference voltage V1 is imposed for comparing a voltage V.sub.2 across the resistor 15, which corresponds to an ion current generated by combustion of the mixture in the combustion chamber 6 and fed to the resistor 15 via the diode 16, as shown by an arrow 17 in FIG. 2, with the reference voltage V.sub.1. The second input terminal of the comparator 11 is connected through a resistor 13 to a junction B between the resistor 15 and the DC power source 14. A resistor 12 is connected at its one end to a node between the resistor 13 and the second input terminal of the comparator 11, and at its other end to a power supply. When the voltage V.sub.2 across the ion current sensing resistor 15 as applied to the first input terminal of the comparator 11 becomes higher than the reference voltage V.sub.1 applied to the second input terminal thereof, the comparator 11 generates an output signal which is fed to the signal processor 10 which then generates an ion current sensing signal 19 to the ECU 3. In this connection, the reference voltage V.sub.1 is expressed as follows: EQU V.sub.1 =V.sub.0 +(V.sub.cc -V.sub.0).times.R.sub.2 /(R.sub.1 +R.sub.2)
where V.sub.0 is the voltage at the junction B; V.sub.cc is the source voltage of the power supply connected to the resistor 12; R.sub.1 is the resistance of the resistor 12; and R.sub.2 is the resistance of the resistor 13.
In operation, when the power transistor 2 is turned off by a control signal from the ECU 3, there is developed a high negative voltage 20 across the secondary winding of the ignition coil 1, which is supplied via the diode 4 to the spark plug 5 whereby a spark is generated between the electrodes 5a of the spark plug 5 to fire an air/fuel mixture in the combustion chamber 6 in the cylinder 7. On this occasion, since the high voltage 20 thus generated across the secondary winding of the ignition coil 1 is negative, it is interrupted by the diode 16 and does not transmit to the ion current sensing device 9. During combustion of the mixture in the combustion chamber 6, an ion current is generated between the electrodes 5a of the spark plug 5 and it is fed via the diode 16 to the ion current sensing unit 9, as shown by the arrow 17 in FIG. 2, since an input terminal (or junction A) of the device 9 is biased to a negative voltage by the DC power source 14. As a consequence, there develops a voltage V.sub.2 across the resistor 15 which is fed to the first input terminal of the comparator 11 where it is compared with the reference voltage V.sub.1 applied to the second input terminal thereof. When the voltage V.sub.2 becomes higher than the reference voltage V.sub.1, the comparator 11 generates an output signal which is then fed to the signal processor 10. The signal processor 10 processes the output signal from the comparator 11 and generates an ion current sensing signal 19 to the ECU 3. Based on this signal 19 as well as a crank angle signal from the signal generator 8, the ECU 3 determines that normal combustion has taken place in the cylinder 7.
With the known ion current sensing device as constructed above, if carbon, other combustion products and the like are deposited or accumulated on the surfaces of the electrodes 5a of the spark plug 5 during an extended period of engine operation, the electrodes 5a become subject to a negative bias voltage from the DC power source 14, and a limited leakage current thus flows from the positive terminal of the DC power source 14 to the negative terminal thereof through the electrodes 5a of the spark plug 5, the diode 16 and the resistor 15. As a result, a voltage is always developed across the resistor 15, so that the condition of V.sub.2 &gt;V.sub.1 is always established. Thus, it follows that the comparator 11 malfunctions to generate an output signal at all times irrespective of the presence or absence of an ion current, considerably impairing the operational reliability of the ion current sensing device 9.